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<h1><a name="SECTION0033400000000000000000">
31.4 Using Visitors to Walk ASTs</a>
</h1>

<p>
<a name="module-compiler.visitor"></a>
<p>
The visitor pattern is ...  The <tt class="module"><a href="module-compiler.html">compiler</a></tt> package uses a
variant on the visitor pattern that takes advantage of Python's
introspection features to eliminate the need for much of the visitor's
infrastructure.

<p>
The classes being visited do not need to be programmed to accept
visitors.  The visitor need only define visit methods for classes it
is specifically interested in; a default visit method can handle the
rest. 

<p>
XXX The magic <tt class="method">visit()</tt> method for visitors.

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-5504' xml:id='l2h-5504' class="function">walk</tt></b>(</nobr></td>
  <td><var>tree, visitor</var><big>[</big><var>, verbose</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-5505' xml:id='l2h-5505' class="class">ASTVisitor</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>

<p>
The <tt class="class">ASTVisitor</tt> is responsible for walking over the tree in the
correct order.  A walk begins with a call to <tt class="method">preorder()</tt>.  For
each node, it checks the <var>visitor</var> argument to <tt class="method">preorder()</tt>
for a method named `visitNodeType,' where NodeType is the name of the
node's class, e.g. for a <tt class="class">While</tt> node a <tt class="method">visitWhile()</tt>
would be called.  If the method exists, it is called with the node as
its first argument.

<p>
The visitor method for a particular node type can control how child
nodes are visited during the walk.  The <tt class="class">ASTVisitor</tt> modifies
the visitor argument by adding a visit method to the visitor; this
method can be used to visit a particular child node.  If no visitor is
found for a particular node type, the <tt class="method">default()</tt> method is
called. 
</dl>

<p>
<tt class="class">ASTVisitor</tt> objects have the following methods:

<p>
XXX describe extra arguments

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-5506' xml:id='l2h-5506' class="method">default</tt></b>(</nobr></td>
  <td><var>node</var><big>[</big><var>, ...</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-5507' xml:id='l2h-5507' class="method">dispatch</tt></b>(</nobr></td>
  <td><var>node</var><big>[</big><var>, ...</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-5508' xml:id='l2h-5508' class="method">preorder</tt></b>(</nobr></td>
  <td><var>tree, visitor</var>)</td></tr></table></dt>
<dd>
</dl>

<p>

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